Such a circuit is known from Electronics Letters of Feb. 28, 1991, Vol. 27, No. 5, pages 421 to 423. It is an LC component, whose capacitance is formed by coupling a capacitor in parallel with the capacitance of a variable capacitance diode (called "varactor"). Its capacitance depends on the blocking voltage existing therein, which is composed of a suitable bias voltage, and the signal voltage at the inlet to the circuit. Due to the dependence of the capacitance on the signal voltage, the signals have a voltage-dependent delay.
Tests have shown that sufficient equalization, which does not attenuate the signals too much, is only possible with the known LC component if the signals to be equalized are not too large, on the one hand, and the bandwidth of the signals to be transmitted is not too large, on the other. The nonlinear distortions being experienced by a signal that is optically transmitted over an optical fiber path, and which are to be equalized by the named circuit device, are caused to a large extent by the so-called "laser chirp", i.e. a spurious wave length fluctuation of the optical signal, which is a function of the amplitude of the electrical signal to be optically transmitted, and the waveguide dispersion of the optical fiber being used, and its length. The greater this effect, the larger the nonlinear distortions experienced by the signal.
It was shown that a satisfactory signal transmission is possible with a laser emitting at a wavelength of 1550 nm, which has a relatively small "laser chirp", with a standard single-mode optical fiber and an equalizer of the above named type, if the length of the optical fiber is not longer than 12 km and the bandwidth of the signals to be transmitted is not larger than 450 MHz. If signals are to be equalized, which have traversed a longer than 12 km optical fiber length, the known equalizer only offers the possibility of adjusting the bias of the capacitance diode correspondingly low, which, however, lowers the limit frequency of the equalizer, i.e. the maximum frequency of the signals transmitted without any attenuation. In other words: if the known equalizer is able to equalize considerable signal distortions, its limit frequency is so low, that it is too small to transmit broadband signals, such as e.g. the signals of the cable TV frequency band, which extend to 450 MHz.